REFAC: remove unnecessary calls to c_str

Summary

Issue #513 points out that calls to c_str are getting more dangerous with C++11 and can causes crashes. We need to identify incorrect uses. But the problem is that there are currently >1500 calls to c_str in OTB, making it difficult to find those potential dangling pointers. However in a lot of cases they are not necessary because there exists a function that takes a std::string directly. We are further helped by C++11 which added some constructors and methods that take strings, where previously only the const char* version existed.

This MR removes 755 calls to c_str which can simply be deleted and need no other refactoring, because a string overload exists. Other instances will require a bit more manual refactoring so they can be done in a future MR. This is only for the very easy ones that can be fixed with sed.